1. Field of Invention
This invention relates to positioning systems, specifically to an improved absolute positioning system where one or more anchor points are tracked and position is determined by triangulation from the anchor points.
2. Prior Art
Numerous methods have been proposed to position a moving object with respect to a fixed frame of reference. One of the early systems, celestial navigation, is a position fixing technique that was devised to help sailors cross the oceans. Celestial navigation uses angular measurements between common celestial objects or to the horizon. The Sun and the horizon are most often measured. Skilled navigators can use the moon, planets or one of 57 navigational stars whose coordinates are tabulated in nautical almanacs.
Modern versions of celestial navigation are used in satellites to track their angular orientation by comparing images taken by the satellite to a star map programmed into the memory of the satellite mounted stellar sensor. By identifying the stars observed and determining the relative orientation of navigation from the Earth, however, requires that the stars be visible and accuracy is limited (Sextants give position to a 5 kilometer radius).
LORAN is a terrestrial radio navigation system using low frequency radio transmitters that uses multiple transmitters to determine location and/or speed of the receiver. Loran requires an extensive terrestrial transmitter network and reported accuracy is +/−8 meters.
Global Navigation Satellite Systems (GNSS) allow small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a line-of-sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments. In addition to the limited accuracy, GNSS is not capable of determining the attitude (roll, pitch, and yaw) of the receiver, is susceptible to signal blockage (buildings, mountains, etc.) and requires an expensive infrastructure external to the receiver (the satellite network).
GNSS accuracy can be augmented using external information, often integrated into the calculation process, to improve the accuracy, availability, or reliability of the satellite navigation signal. There are many such systems in place and they are generally named or described based on how the GNSS sensor receives the information. The best of these systems is reported to reduce the error to +/−10 cm and requires a more extensive external infrastructure than the satellite network.
Inertial systems comprised of accelerometers and gyroscopes are capable of detecting acceleration in each of the 6 axes (x, y, z, roll, pitch, and yaw) and by taking a double integral of the acceleration data one can calculate the change in position from a starting point. However, each measurement from an inertial sensor represents the change from the previous measurement. Because of this, inertial systems are prone to develop errors caused by random drift (also known as random walk). This drift follows the binomial distribution which says that the expected error is equal to the square root of the number of samples times the average error in a sample. For a system that takes 100 samples a second and has an error of 0.1% per sample, the system has an expected error of 24.5% after 10 minutes.
Positioning the six degrees of freedom to sub-millimeter accuracy, while less important for navigation, is required for many types of motion analysis. Prior art FIG. 1 (US Patent Application No 2009/0063097) shows a system for measuring athletic performance using accelerometers. Sports scientist have made numerous attempts to study athlete movement using a combination of augmented GNSS signals combined with inertial sensor systems like those of FIG. 1 without success due to the above described issues with GNSS and the random drift inherent in inertial sensor systems.
Today, there does not exist a positioning system capable of measuring x, y, z, roll, pitch and yaw to sub-millimeter accuracy without an external infrastructure or a physical connections between the moving object and the fixed frame of reference. For these reasons and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification there is a need in the art for a means to determine the absolute position (x, y, z) and attitude (roll, pitch, yaw) of an object which is free to move in an external reference plane to sub-millimeter accuracy.
3. Objects and Advantages
Accordingly, several objects and advantages of the present invention are:                (1) to provide a system capable of determining the position (x, y, z) and attitude (roll, pitch, and yaw) of a moving object within an external reference frame to sub-millimeter accuracy.        (2) to provide a system capable of determining accurately the position and attitude without a fixed infrastructure of transmitters, targets, or other prearranged devices.        (3) to provide a system capable of determining accurately the position and attitude of the moving object without a physical connection to the external reference frame.        (4) to provide a system capable of determining accurately the position and attitude of a moving object based on absolute measurements (as opposed to a system where each measurement is based on the previous measurement) to avoid the drift issues associated with accelerometers and gyroscopes.        
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing descriptions.